stairs: refactor wool registration, fixes wool:yellow stairs entry

for _,name in ipairs({"red", "green", "white", "black", "blue", "brown", "cyan",

local name = {"red", "green", "white", "black", "blue", "brown", "cyan",

            "grey", "dark_green", "dark_grey", "magenta", "orange", "pink",

                    "red", "violet", "yellow"}) do

            "red", "violet", "yellow"}

for i = 1, 16 do

    local name = name[i]

    local nodename = "wool:"..name

    local tmp = minetest.registered_nodes[nodename]

    if not tmp then